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4. New netfilter targets

In this section, we will attempt to explain the usage of new netfilter targets. The patches will appear in alphabetical order. Additionally, we will not explain patches that break other patches. But this might come later.

Generally speaking, for targets, you can get the help hints from a particular module by typing :

# iptables -j THE_TARGET_YOU_WANT --help

This would display the normal iptables help message, plus the specific ``THE_TARGET_YOU_WANT'' target help message at the end.

4.1 ftos patch

This patch by Matthew G. Marsh <mgm@paktronix.com> adds a new target that allows you to set the TOS of packets to an arbitrary value.

For example, if you want to set the TOS of all the outgoing packets to be 15, you can do as follows :

# iptables -t mangle -A OUTPUT -j FTOS --set-ftos 15

# iptables -t mangle --list
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
FTOS       all  --  anywhere             anywhere           TOS set 0x0f 

Supported options for the FTOS target are :

--set-ftos value

-> Set TOS field in packet header to value. This value can be in decimal (ex: 32) or in hex (ex: 0x20)

4.2 IPV4OPTSSTRIP patch

This patch by Fabrice MARIE <fabrice@celestix.com> adds a new target that allows you to strip all the IP options from an IPv4 packet.

It's simpled loaded as follows :

# iptables -t mangle -A PREROUTING -j IPV4OPTSSTRIP

# iptables -t mangle --list
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination
IPV4OPTSSTRIP  all  --  anywhere             anywhere

This target doesn't support any option.

4.3 NETLINK patch

This patch by Gianni Tedesco <gianni@ecsc.co.uk> adds a new target that allows you to send dropped packets to userspace via a netlink socket.

For example, if you want to drop all pings and send them to a userland netlink socket instead, you can do as follows :

# iptables -A INPUT -p icmp --icmp-type echo-request -j NETLINK --nldrop

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
NETLINK    icmp --  anywhere             anywhere           icmp echo-request nldrop 

Supported options for the NETLINK target are :

--nldrop

-> Drop the packet too

--nlmark <number>

-> Mark the packet

--nlsize <bytes>

-> Limit packet size

For more information on netlink sockets, you can refer to the Netlink Sockets Tour.

4.4 NETMAP patch

This patch by Svenning Soerensen <svenning@post5.tele.dk> adds a new target that allows you create a static 1:1 mapping of the network address, while keeping host addresses intact.

For example, if you want to alter the destination of incoming connections from 1.2.3.0/24 to 5.6.7.0/24, you can do as follows :

# iptables -t nat -A PREROUTING -d 1.2.3.0/24 -j NETMAP --to 5.6.7.0/24

# iptables -t nat --list
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination         
NETMAP     all  --  anywhere             1.2.3.0/24         5.6.7.0/24

Supported options for NETMAP target are :

--to address[/mask]

-> Network address to map to.

4.5 SAME patch

This patch by Martin Josefsson <gandalf@wlug.westbo.se> adds a new target which is similar to SNAT and will gives a client the same address for each connection.

For example, if you want to modify the source address of the connections to be 1.2.3.4-1.2.3.7 you can do as follows :

# iptables -t nat -A POSTROUTING -j SAME --to 1.2.3.4-1.2.3.7

# iptables -t nat --list
Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination         
SAME       all  --  anywhere             anywhere           same:1.2.3.4-1.2.3.7 

Options supported by the SAME target are :

--to <ipaddr>-<ipaddr>

-> Addresses to map source to. May be specified more than once for multiple ranges.

--nodst

-> Don't use destination-ip in source selection

4.6 tcp-MSS patch

This patch by Marc Boucher <marc+nf@mbsi.ca> adds a new target that allows you to examine and alter the MSS value of TCP SYN packets, to control the maximum size for that connection.

As explained by Marc himself, THIS IS A HACK, used to overcome criminally brain-dead ISPs or servers which block ICMP Fragmentation Needed packets.

Typical usage would be :

# iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

# iptables --list
Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
TCPMSS     tcp  --  anywhere             anywhere           tcp flags:SYN,RST/SYN TCPMSS clamp to PMTU 

Options supported by the tcp-MSS target are (mutually-exclusive) :

--set-mss value

explicitly set MSS option to specified value

--clamp-mss-to-pmtu

automatically clamp MSS value to (path_MTU - 40)

4.7 TTL patch

This patch by Harald Welte <laforge@gnumonks.org> adds a new target that enables the user to set the TTL value of an IP packet or to increment/decrement it by a given value.

For example, if you want to set the TTL of all outgoing connections to 126, you can do as follows :

# iptables -t mangle -A OUTPUT -j TTL --ttl-set 126

# iptables -t mangle --list
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
TTL        all  --  anywhere             anywhere           TTL set to 126 

Supported options for the TTL target are :

--ttl-set value

-> Set TTL to <value>

--ttl-dec value

-> Decrement TTL by <value>

--ttl-inc value

-> Increment TTL by <value>

4.8 ulog patch

This patch by Harald Welte <laforge@gnumonks.org> adds a new target which supplies a more advanced packet logging mechanism than the standard LOG target. The `libipulog/' contains a library for receiving the ULOG messages.

Harald maintains a web page containing the proper documentation for ULOG, so there is no point for me to explain this here..